1. Technical Field
The present invention relates to a synchronous rectifying DC-DC converter.
2. Background Art
FIG. 1 shows two types of synchronous rectifying DC-DC converter circuit. Herein, FIG. 1(a) shows a step-down DC-DC converter and FIG. 1(b) shows a step-up DC-DC converter. Both circuits are simply illustrated so that a concept of an operation can be seen. A smoothing capacitor is not shown in the figures.
As shown in FIG. 1(a), in the step-down DC-DC converter, one end of a switching element Q1 connects to a voltage input terminal Vin and the other end connects to a voltage output terminal Vout via a choke coil L1. A node between the switching element Q1 and the choke coil L1 is grounded via a switching element Q2. In this case, the switching element Q1 serves as a main switching element and the switching element Q2 serves as a synchronous rectifying switching element playing a role of a flywheel diode.
In the step-down DC-DC converter having the above-described configuration, when an input voltage vi is applied to the voltage input terminal Vin and when the switching elements Q1 and Q2 are alternately turned ON and OFF with an ON duty ratio of the switching element Q1 being set to D (0<D<1), an output voltage vo is output to the voltage output terminal Vout in accordance with a relational expression vo=vi×D. In the following description, “duty ratio” means an ON duty ratio (duty cycle). When an OFF duty ratio is to be referred to, that will be clearly specified.
On the other hand, as shown in FIG. 1(b), in the step-up DC-DC converter, one end of a choke coil L1 connects to a voltage input terminal Vin and the other end connects to a voltage output terminal Vout via a switching element Q1. A node between the choke coil L1 and the switching element Q1 is grounded via a switching element Q2. In this case, the switching element Q2 serves as a main switching element and the switching element Q1 serves as a synchronous rectifying switching element.
In the DC-DC converter having the above-described configuration, when an input voltage vi is applied to the voltage input terminal Vin and when the switching elements Q1 and Q2 are alternately turned ON and OFF while an ON duty ratio of the switching element Q1 is set to D (0<D<1), an output voltage vo is output to the voltage output terminal Vout in accordance with a relational expression vo=vi÷D.
Each of the above-described circuits is based on the assumption that the efficiency is 100%. Actually, dead time when the two switching elements are OFF is required in order to prevent a short circuit, and means for realizing that configuration may be required. However, that point is not essential to the present invention, and thus is not described here.
As can be understood by comparing the circuit configurations of the two types of DC-DC converter shown in FIG. 1, the step-down circuit is equivalent to the step-up circuit when viewed from the output side to the input side, although the roles of the main switching element and the synchronous rectifying switching element change.
In a diode rectifying method, a diode is used instead of a synchronous rectifying switching element, for example, a diode is used instead of the switching element Q2. In this case, a step-down circuit is not equivalent to a step-up circuit even when viewed from the output side to the input side because a reverse current cannot flow through the diode.
In a DC-DC converter, a set value of an output voltage is slowly raised in many cases so that a large current to charge a smoothing capacitor (output capacitance) from zero does not flow through each switching element at startup. This is called soft start control. For example, when the soft start control is performed in a step-down DC-DC converter, a duty ratio of a main switching element (the switching element Q1 in the DC-DC converter shown in FIG. 1) is gradually increased from zero to a value that depends on input and output voltages.
In recent years, various types of ICs require a plurality of power supply voltages due to the high performance thereof. Accordingly, power supply voltages are supplied to an IC by connecting a plurality of DC-DC converters of different output voltages to the IC in some cases. In those cases, depending on the order of applying the respective power supply voltages, that is, depending on the order of starting the respective DC-DC converters, a voltage from one power supply (DC-DC converter) may leak to a terminal to which a power supply voltage from another DC-DC converter is to be applied. This means that a DC voltage is applied in advance to an output terminal of the DC-DC converter that is still to be started. This state is called a pre-bias state. A voltage applied to the output terminal of the DC-DC converter before startup is called a pre-bias voltage.
As described above, when soft start control is performed in the step-down DC-DC converter, the ON duty ratio of the main switching element (switching element Q1) is gradually increased from zero. When viewed from the output side to the input side, this is equivalent to gradually increasing the duty ratio of the synchronous rectifying switching element (switching element Q1) from zero in the step-up DC-DC converter (while gradually decreasing a large duty ratio in the main switching element). Thus, voltage rises by 1/D of the voltage of the voltage output terminal just after startup, and the risen voltage appears in the input side.
For example, in a step-down DC-DC converter having an input voltage of 5 V and an output voltage of 2.5 V, assume that a pre-bias voltage of 1 V exists at the voltage output terminal and that D=0.05 just after startup. In that case, a voltage of 1÷0.05=20 V occurs at the input side because of the step-up operation. This voltage is four times the input voltage at that time, so that the DC-DC converter can be destroyed or an overvoltage protecting circuit can malfunction.
Such an operation of returning energy from the output side to the input side is called a regenerative operation. The regenerative operation is normally performed during normal operation when a load current is small (load is light) in a synchronous rectifying DC-DC converter. However, since the ON duty ratio of the main switching element depends on input and output voltages during normal operation, an extreme step-up operation from the output side to the input side is not performed. Two documents of background interest are Patent Document 1: Japanese Unexamined Patent Application Publication No. 2003-259627; and Nonpatent Document 1: TPS40001 data sheet, Texas Instruments Incorporated, incorporated by reference.
Patent Document 1 discloses a circuit to solve the above-described problem of the regenerative operation at the soft start. In the DC-DC converter disclosed in Patent Document 1, a direction of a current flowing through a choke coil is observed, and a synchronous rectifying switching element is turned OFF upon detection of a reverse current. Accordingly, a reverse current does not flow through the synchronous rectifying switching element. That is, the regenerative operation is completely prohibited. Therefore, this DC-DC converter, which is a synchronous rectifying type, substantially functions as a constant diode-rectifying DC-DC converter. Although not directly presented as a problem in Patent Document 1, this configuration prevents a pre-bias voltage from rising and existing at the input side at startup.
Also, Nonpatent Document 1 discloses a circuit to solve the above-described problem. In the DC-DC converter disclosed in Nonpatent Document 1, the same control as that in the DC-DC converter disclosed in Patent Document 1, that is, control for preventing a reverse current from flowing through a synchronous rectifying switching element, is performed only at startup by the soft start function, and the synchronous rectifying switching element is allowed to perform the normal operation of a synchronous rectifying circuit after the starting period has passed. In this case, too, the DC-DC converter substantially functions as a diode-rectifying DC-DC converter at startup, and thus a pre-bias voltage can be prevented at the input side.
The DC-DC converter disclosed in Patent Document 1 is based on the assumption that no regenerative operation is performed because a predetermined load current stably flows during a normal operation, and thus no substantial problem will occur even if the regenerative operation is prohibited. However, a problem arises if the DC-DC converter is used under a condition where a load current significantly varies during a normal operation. That is, an output voltage significantly varies if a load current varies during a normal operation.
For example, assume a case where a large load current rapidly becomes small. At this time, an output voltage transiently rises. Upon detecting the rise of the output voltage, the DC-DC converter tries to drop the output voltage by decreasing the ON duty ratio of the main switching element and by increasing the ON duty ratio of the synchronous rectifying switching element. In a typical synchronous rectifying DC-DC converter, a reverse current flows through a choke coil, a synchronous rectifying switching element, and a main switching element, so that electric power can be regenerated in the input side. Accordingly, a rise of the output voltage can be suppressed and a predetermined voltage can be recovered in a short time. However, the DC-DC converter disclosed in Patent Document 1 substantially functions as a diode-rectifying DC-DC converter, and it is constantly prohibited that a reverse current flows through a choke coil. This causes a problem that an output voltage is kept high for a long time. This problem occurs because a regenerative operation is prohibited during a normal operation.
On the other hand, the DC-DC converter disclosed in Nonpatent Document 1 functions as a synchronous rectifying DC-DC converter during a normal operation. Thus, the regenerative operation can be performed, and the problem in the circuit according to Patent Document 1 does not occur.
However, another problem exists: depending on a condition of an output current, an output voltage significantly drops at transition from a diode rectifying operation state (a state where a reverse current in a choke coil is prohibited) during a starting period with soft start, to a normal synchronous rectification state (a state where a reverse current is permitted or a regenerative operation is permitted).
For example, assume a case where a load current is very small during a starting period. As described above, the DC-DC converter substantially operates as a diode rectifying DC-DC converter because the regenerative operation is prohibited during the starting period. In this case, the ON duty ratio of the main switching element does not depend on a difference between input and output voltages, unlike in a case where the regenerative operation can be performed, but may be a duty ratio capable of supplying power required by the DC-DC converter to maintain its own operation. Thus, the ON duty ratio of the main switching element is very small, so that the main switching element is in an ON-state for only a short time in one period. For example, when an input voltage is 5 V and when an output voltage is 2.5 V, the ON duty ratio of the main switching element should be D=0.5 in a normal synchronous rectifying operation state. However, a state where D=0.01 continues if the regenerative operation is prohibited.
After the starting period ends and a normal synchronous rectification state where a reverse current in a choke coil is permitted occurs, the ON duty ratio of the main switching element changes from a very small ratio to an original ratio, so that an output voltage at once drops significantly. A control system detects the drop of the output voltage and tries to increase the duty ratio so that the output voltage reaches a predetermined value. However, the drop of the output voltage continues during the response time. This problem is also due to prohibition of the regenerative operation, although only during a starting period.